There are command line options:
bitcoin -addnode=1.2.3.4 to tell bitcoin about a node to connect to
bitcoin -connect=1.2.3.4 connect only to the specified node(s)
You can use more than one of these, for instance
bitcoin -connect=(first to try) -connect=(next to try) ...
You can specify non-routable IPs with -connect like 192.168.x.x, so if you had a server farm and you wanted one server to connect to the world and the rest to connect to the one server, you could do that.
In particular, -addnode is needed if you're always going to connect through TOR, since the IRC server blocks all the TOR exit nodes. To connect through TOR, you could use:
bitcoin -proxy=127.0.0.1:9050 -addnode=212.159.72.216